1. Field of the Invention
The present invention is related to a method for detecting the center of target marks by image processing, wherein the center position of the target marks may be provided by such means as printing on such workpiece as printed circuit board for boring on it.
2. Background and Material Information
In workpieces, such as printed circuit boards, in order to bore reference holes, target marks made of copper foil print are provided on four corners of the workpiece. Then, the workpiece is moved by a workpiece driving apparatus onto a table, such that each of the target marks come in position just beneath an imaging station. Also, the center position of each target mark is detected by any one of a predetermined means. The boring machine is slightly moved (movement for correction) in the directions of the X and Y axes with an X and Y axis driving mechanism, respectively, by the amount of difference between the detected center position of the target mark and the drilling position, to align the detected center position with the center of the drilling device. Next, the drilling device is moved up by a Z axis driving mechanism to bore a reference hole in the center which coincides with the center of the target mark.
At this point, it is worth noting that since the reference hole is provided to determine the reference position when, for example, circuits are printed on a workpiece, it is required for the reference hole to be bored at the theoretically correct position.
The center of the target marks has been determined by analog to digital (A/D) converting a monochrome image signal to a digital signal at the A/D converter, as well as by converting the digital signal to binary data at an image processor to display on a monitor (CRT) to measure the gravity center using a dynamic window method.
The target marks are formed, for example, by printing round copper foil, or by providing a plurality of concentric circles by printing circular copper foil on the workpiece between the center portion of the substrate of the workpiece and the peripheral portion of the substrate of the workpiece.
For the measurement of the gravity center, if any pattern other than the target marks was converted to black-and-white image by binary conversion, that pattern may significantly affect the coordination of black-and-white conversion, so that problems arise with the accuracy of the detection of the center of target marks.